vscode.debug.activeDebugSession is updated asynchronously

[hbenl]

• VSCode Version: 1.32.1, 1.33.0-insider from 2019-03-08T06:57:33.004Z
• OS Version: Arch Linux, Windows 10, MacOS

In 2 of my extensions, I start a debug session and then try to read vscode.debug.activeDebugSession, like this:

await vscode.debug.startDebugging(workspaceFolder, debugConfig);
const session = vscode.debug.activeDebugSession;

Starting with version 1.32.1, the session I get is undefined. However, I found a very simple workaround:

await vscode.debug.startDebugging(workspaceFolder, debugConfig);
await new Promise(resolve => setImmediate(resolve));
const session = vscode.debug.activeDebugSession;

So it seems that vscode.debug.activeDebugSession is updated asynchronously immediately after the promise from vscode.debug.startDebugging() is resolved.

I think that this is probably related to #69534, maybe the fix for that issue will also solve this one. I will check this as soon as there is an Insiders build containing that fix.

[hbenl]

I just checked with the Insiders version from 2019-03-11T08:42:16.804Z and the bug is still there.

[weinand]

This issue and #69534 are probably originating from the same change: some internal method has become asynchronous. This was breaking the vscode.debug.onDidStartDebugSession and we had to fix it.

But vscode.debug.activeDebugSession is not broken by this: you cannot assume that vscode.debug.activeDebugSession is updated immediately after you are calling startDebugging. Previously you might have been able to observe this, but there is no guarantee for this.

A better approach would be to register a vscode.debug.onDidChangeActiveDebugSession handler.
But be aware: the debug session that you receive via onDidChangeActiveDebugSession is not necessarily the one you've started with startDebugging.

A safe way to find the correct debug session is to pass a magic property to startDebugging and then listen on vscode.debug.onDidStartDebugSession and trying to find the magic property in the session's configuration...

[hbenl]

I am not assuming that vscode.debug.activeDebugSession is updated immediately after calling startDebugging. But since startDebugging returns a Promise one would assume that it is resolved after the asynchronous operation is finished, not before.
The way it is now, I find the API to be misleading. What's the point of returning a Promise if it doesn't signify anything useful? I think the Promise should either be removed or made to work as one would expect.
I know how to work around this, so it's not a big issue, but I'm sure others will be confuses by this as well.

[weinand]

startDebugging returns a promise that resolves to true when debugging could be successfully started. It does not say anything about the value of activeDebugSession.

The activeDebugSession is driven by the UI (and the users). It is asynchronous by nature and unrelated to startDebugging.